1. Field of the Invention
The invention relates to defoamers for aqueous media, comprising as hydrophobic solids critically influencing the defoaming certain urea derivatives which acquire the properties requisite for their particular activity as defoamers by crystallization from a clear melt dispersed homogeneously in the carrier medium.
2. Related Art
In many industrial processes, and especially when working in aqueous media, it is necessary to suppress, or prevent entirely, the unwanted formation of foam during the preparation or processing operations. This can be achieved by adding what are known as antifoams or defoamers, which even when used at very low concentrations upward of about 0.001% by weight are able to prevent or destroy unwanted foams. Examples of such prior art defoamers are silicone oils, mineral oils, hydrophobic polyoxyalkylenes, long-chain alcohols, and also mixtures of these products with one another and emulsions thereof. To reinforce the activity, it is common to add hydrophobic solids in amounts of from 0.1 to 10% by weight, which specifically promote dewetting processes on foam lamellae and therefore very actively assist in foam collapse. Suitable hydrophobic solids are appropriate silicas, metal stearates, polyolefins, and waxes.
The use of urea and urea derivatives as additives to defoamer formulation is also known per se.
European Patent No. EP-A-0 115 585, which corresponds to U.S. Pat. No. 4,696,761, describe ureas which are prepared in situ in an organic carrier medium at relatively low temperatures and which have defoaming properties for aqueous media. They are obtained by combining preferably equivalent amounts of isocyanates and amines in the organic carrier medium in question at temperatures below the melting point of the reaction product.
This gives urea derivatives of the general formula 
R=alkyl C4-C30 
Rxe2x80x2=single chemical bond; alkylene C2-C12, mono- to dinuclear aryl radicals which have additional alkyl groups C1-C9 on the aryl radical; cycloalkylene
Rxe2x80x3=H, alkyl C1-C24 
Rxe2x80x2xe2x80x3=H, xe2x80x94CH3 
x=0.5.
It is expressly emphasized that when heated beyond their melting point, or when prepared at above their melting point, the ureas possess only an insignificant defoaming action. This is correlated with the formation of monodisperse or micellar structures during the in situ formation of the urea derivatives.
A disadvantage here is that, as a result of the in situ formation of the urea derivatives in accordance with the procedure described in EP-A-0 115 585, it is necessary to resort to carrier media which cannot react with amines and in particular not with isocyanates either. For example, the hydroxy-functional polyoxypropylenes well-known to the skilled worker as carrier media for defoamers, and hydroxy-functional polyoxyalkylene-polysiloxanes as well, are rejected on account of their hydroxyl functionality. It is true that, in principle, the reaction of an amine with an isocyanate is clearly preferred over the reaction of a hydroxy compound with an isocyanate, in respect of the reaction rate; however, and especially in the presence of the amines, which are known to be catalysts for the nucleophilic addition of hydroxy compounds onto isocyanates, hydroxy compounds react as well to uncontrollable extents and so result in urea derivatives which it is difficult, if possible at all, to crystallize. This would of course directly affect the activity of the resulting urea derivatives and would result at least in results that are difficult if not impossible to reproduce in the context of their use as defoamers.
This invention provides a composition for defoaming aqueous media, comprising as defoamers urea derivatives of the formula I 
where
R1xe2x80x94is a hydrocarbon radical or a hydrocarbon radical and one nitrogen atom or a hydrocarbon radical and one carbonyl group,
R2xe2x80x94is a hydrogen atom or a hydrocarbon radical,
R3xe2x80x94is a hydrogen atom or a hydrocarbon radical,
R4xe2x80x94is an organic radical, and
nxe2x80x94is from 0 to 5; in the form of solid particles, which are obtained by crystallization from a clear homogeneous melt dispersed in a carrier medium.
This invention also provides a process for preparing a solid particle urea derivative of the formula I 
where
R1xe2x80x94is a hydrocarbon radical or a hydrocarbon radical having one nitrogen atom or a hydrocarbon radical having one carbonyl group,
R2xe2x80x94is a hydrogen atom or a hydrocarbon radical,
R3xe2x80x94is a hydrogen atom or a hydrocarbon radical,
R4xe2x80x94is an organic radical, and
nxe2x80x94is from 0 to 5;
comprising the steps of
(a) heating the urea derivative solution in a first carrier media above the melting points of said urea derivative to form a homogeneous solution and
(b) mixing the homogeneous solution of step (a) with a second carrier media having a temperature of less than 25xc2x0 C.
It has surprisingly now been found that, for the preparation of defoamer formulations containing urea derivatives as hydrophobic solids, the above-described in situ preparation of such urea derivatives below their melting point is neither necessary nor advantageous and that, instead, by means of controlled melting and recrystallization processes, it is possible to prepare defoamer formulations having improved properties and which, indeed, through the methodical variation of melting and recrystallization conditions, permit custom-tailored property profiles. By preparing the urea derivatives separately it is also possible, if required, to use exclusively carrier media which on account of their potential reactivity with respect to amines and/or isocyanates, because of the side reactions described above, are not amenable to precipitative crystallization in the manner described in EP-A-0 115 585. These include, for example, hydrophobic polyoxyalkylenes and organomodified siloxanes, which may also contain hydroxy functions.
This procedure is specifically successful even with urea derivatives which are explicitly described in EP-A-0 115 585 and which therefore, in accordance with the prior art, in the case of the process described in this patent, should no longer show any extraordinary activity in defoamer formulations. Such urea derivatives are easy to prepare from the corresponding isocyanates and amines.
The invention therefore provides compositions for defoaming aqueous media, comprising as defoamers urea derivatives of the formula I 
where
R1xe2x80x94is a hydrocarbon radical, preferably having 4 to 30 carbon atoms, or a hydrocarbon radical, preferably having 4 to 24 carbon atoms, and one nitrogen atom or a hydrocarbon radical, preferably having 4 to 30 carbon atoms and one carbonyl group,
R2xe2x80x94is a hydrogen atom or a hydrocarbon radical, preferably having 1 to 24 carbon atoms,
R3xe2x80x94is a hydrogen atom or a hydrocarbon radical, preferably having 1 to 24 carbon atoms,
R4xe2x80x94is an organic radical, preferably having 2 to 30 carbon atoms, and
nxe2x80x94is from 0 to 5; in the form of solid particles, which are obtained by crystallization from a clear homogeneous melt dispersed in a carrier medium.
Preferred compositions are those in which R1 is a hydrocarbon radical having 4 to 24 carbon atoms, R2 is a hydrogen atom, R3 is a hydrogen atom, and R4 is a hydrocarbon radical having 2 to 24 carbon atoms.
Preference is further given to those compositions of the invention in which R1 is the organic radical 
where R5 is a hydrocarbon radical having 1 to 18 carbon atoms, R2 and R3 are a hydrogen atom, and R4 is a hydrocarbon radical having 2 to 24 carbon atoms.
In addition, those compositions of the invention comprising as defoamers urea derivatives in which R1 is the organic radical 
where R6 is an organic radical having 2 to 30 carbon atoms,
R2 and R3 are a hydrogen atom, and R4 is a hydrocarbon radical having 2 to 24 carbon atoms, and compositions comprising as defoamers urea derivatives in which R1 is a hydrocarbon radical having 2 to 24 carbon atoms, R2 and R3 are a hydrogen atom, and R4 is the organic radical 
where R7 is a hydrocarbon radical having 2 to 20 carbon atoms, are preferred compositions according to the present invention.
This invention also provides a cooling lubricant, a polymer dispersion, a coating material or a printing ink, comprising compositions for defoaming aqueous media wherein the compositions further comprise as defoamers urea derivatives of the formula I 
where
R1xe2x80x94is a hydrocarbon radical, preferably having 4 to 30 carbon atoms, or a hydrocarbon radical, preferably having 4 to 24 carbon atoms, and one nitrogen atom or a hydrocarbon radical, preferably having 4 to 30 carbon atoms, and one carbonyl group,
R2xe2x80x94is a hydrogen atom or a hydrocarbon radical, preferably having 1 to 24 carbon atoms,
R3xe2x80x94is a hydrogen atom or a hydrocarbon radical, preferably having 1 to 24 carbon atoms,
R4xe2x80x94is an organic radical, preferably having 2 to 30 carbon atoms, and
nxe2x80x94is from 0 to 5;
in the form of solid particles, which are obtained by crystallization from a clear homogeneous melt dispersed in a carrier medium.
This invention further provides a process for preparing a solid particle urea derivative of the formula I 
where
R1xe2x80x94is a hydrocarbon radical, preferably having 4 to 30 carbon atoms, or a hydrocarbon radical, preferably having 4 to 24 carbon atoms and one nitrogen atom, or a hydrocarbon radical, preferably having 4 to 30 carbon atoms and one carbonyl group,
R2xe2x80x94is a hydrogen atom or a hydrocarbon radical, preferably having 1 to 24 carbon atoms,
R3xe2x80x94is a hydrogen atom or a hydrocarbon radical, preferably having 1 to 24 carbon atoms,
R4xe2x80x94is an organic radical, preferably having 2 to 30 carbon atoms, and
nxe2x80x94is from 0 to 5
which
comprising the steps of
(a) heating the urea derivative solution in a first carrier media above the melting points of said urea derivative to form a homogeneous solution and
(b) mixing the homogeneous solution of step (a) with a second carrier media having a temperature of less than 25xc2x0 C.
For the present invention, therefore, it is completely irrelevant whether the urea derivatives of the invention are prepared separately and introduced as solids or are generated in the form of a precipitative crystallization in a liquid phase. An important feature at this point, in contrast, are the melting and recrystallization processes, which for the easy-to-prepare urea derivatives described herein result in surprising properties as hydrophobic solids in defoamers.
To this end, the recrystallization of the melted urea derivatives must take place from a clear melt dispersed homogeneously in the carrier medium, in order to reach optimum activity. This is easily possible by varying the main parameters, such as the chemical nature of the carrier oil, the urea derivative, and the temperature.
If the urea derivatives are merely melted in the carrier medium to form a non-homogenous mixture, i.e. not a clear mixture, the process of the invention cannot be used to prepare defoamers having sufficient activity.
The practical implementation of the above-described melting and recrystallization processes can be carried out by various methods. For example, the crystallization of the urea derivative melted in a carrier medium A can be influenced within a wide range, depending on the choice of the temperature gradients and the nature and extent of the shear energy employed, by addition of or addition to a carrier medium B of defined temperature. In this context, carrier media A and B can be identical or different.
Likewise, the melts of the urea derivatives can also be induced to crystallize in a particular way by measures ranging from simply leaving them to stand at room temperature through to the supply of external cooling by means of coolants. Here again, the chosen circumstances define the crystallization form of the urea derivatives and thus the activity of the resultant defoamer formulations. For instance, the recrystallization of the melted urea derivatives can also be carried out with advantage in the presence of additional solids, which can serve as crystallization nuclei, in order to obtain particular desired crystal morphologies.
In order to obtain urea derivatives which are as effective as possible in terms of the present invention, it is important to form particularly small, spherical urea crystals. This can be specifically influenced by varying, for example, the concentration of the urea derivatives in the melt. The smaller the chosen concentration of the urea derivatives in the melt, the smaller the crystals that can be produced. Furthermore, the formation of small urea derivative crystals is favored by a large, rapidly traversed temperature gradient, when, for example, a hot melt is shock-cooled by pouring it into a much colder carrier medium. High stirring outputs with stirrers which ensure effective, thorough mixing prevent the development at this point of undesirably large, persistent concentration gradients, which in turn lead to relatively large crystals and/or agglomerates thereof. It is, however, also possible with advantage initially to establish, at least partially, conditions which ought to promote the formation of relatively small crystals and then to follow these by conditions which promote large crystals. For example, a melt can be cooled rapidly at first and then slowly thereafter. In this way it is ensured that there are numerous crystallization nuclei, formed suddenly, on which, subsequently and more slowly, regularly formed crystals grow.
Suitable carrier media include not only organic or mineral oils but also siloxanes or organomodified siloxanes.
Defoamers of this kind can also be converted into aqueous emulsions by adding emulsifiers.
The defoamers of the invention can be used, for example, to defoam cooling lubricants, polymer dispersions, coating materials, and printing inks.